The present invention relates, generally, to optical fiber-based sensors and, more specifically, to very-high-temperature stable optical fiber-based sensors having internal gratings which differentially diffract light of different wavelengths in a manner related to an environmental parameter.
In recent years, optical fiber-based sensors have become an increasingly accepted alternative to conventional sensing technologies. Optical fiber-based sensors have been developed which allow for highly sensitive measurement of environmental parameters such as temperature, pressure, strain, and chemical concentrations, while offering a high degree of resistance to electromagnetic interference. Also, optical fiber-based sensors may be imbedded in structures to allow in situ measurement of internal environmental parameters which would, otherwise, not be readily measurable. Fiber-based sensors offer many additional advantages over their pneumatic and electronic counterparts including: increased sensitivity, geometrical flexibility, ease of miniaturization, and multiplexing capabilities.
A number of optical fiber-based sensors have been developed to utilize these advantages. Prior optical fiber-based sensors have, generally, been fabricated by writing gratings into selected portions of the core of an optical fiber by exposing those portions to intensely focused light having a wavelength to which the optical fiber is photosensitive. The light photoinduces a refractive index variation onto the selected portions of the fiber to produce a diffraction grating. Two basic types of these gratings are typically employed and include long-period fiber gratings and short period fiber gratings. The long-period fiber gratings (LPFG""s) have a periodicity or spacing of the gratings (50-1500 microns) along the fiber length that is much larger than the wavelength of light source used during operation of the optical fiber. This is in contrast to short-period gratings, also known as fiber Bragg gratings, which have periods that are, generally, less than the wavelength of the light source used during operation of the optical fiber. Optical fiber-based sensors which use short-period fiber Bragg gratings, typically, suffer from the disadvantage of having relatively low sensitivity in comparison to optical fiber-based sensors having long-period fiber gratings.
The most commonly used current method for producing optical fiber-based sensors relies on the use of ultraviolet (UV) radiation to change the local refractive index of a specially-selected photosensitive optical fiber (referred to herein as xe2x80x9cUV-induced optical fiber gratingsxe2x80x99). Germanosilicate glasses have been found to be especially well suited for the writing of gratings therein using UV radiation. Grating patterns are commonly written onto UV-sensitive optical fibers using KrF or ArF excimer lasers operating at respective wavelengths of 193 or 248 nm. Suggested writing mechanisms have included UV absorption by germanium-oxygen vacancy defect centers, stress relief, and densification of the glass. The efficiency of writing onto a UV-sensitive optical fiber may be increased by preloading the optical fiber with molecular hydrogen.
Unfortunately, UV-induced optical fiber gratings have been found to have a limited high-temperature operational lifetime. Further, UV-induced optical fiber gratings are known to degrade as a result of annealing at 300-400 degrees Celsius, and are substantially erased after less than an hour of such annealing. In fact, the degradation of UV-induced optical fiber gratings may be a problem even at room temperature, although pre-annealing may be used to stabilize their properties at room temperature over longer time periods. Such degradation has precluded the use of UV-induced optical fiber gratings in some hostile environments.
Thus, it appears that none of the prior art optical fiber-based sensors are ideally suited for use in hostile, high-temperature environments. Therefore, there exists a need in the industry for an optical fiber-based apparatus and method of measuring environmental parameters in high-temperature environments and a method for manufacturing an optical fiber-based sensor for use therein, which address these and other related, and unrelated, problems.
Broadly described, the present invention comprises an optical fiber-based apparatus, or system, for measuring an environmental parameter in a high-temperature environment, a method of using the optical fiber-based apparatus for such measurement, and a method of manufacturing the optical fiber-based apparatus. More particularly and in accordance with exemplary embodiments, the optical fiber-based apparatus comprises an optical fiber-based sensor including a silica-based optical fiber which has a plurality of thermally-induced, long-period fiber gratings disposed thereon at appropriate locations along the longitudinal axis of the optical fiber. The gratings exhibit exceptional thermal stability and do not degrade at temperatures in excess of 600 degrees Celsius or in excess of 1200 degrees Celsius, thereby enabling the optical fiber-based sensor of the present invention to be used for measuring environmental parameters such as temperature, pressure, strain, or chemical composition, in high-temperature environments.
The optical fiber-based apparatus further comprises a light source operatively coupled to one end of the optical fiber-based sensor and a detector operatively coupled to the other end thereof. Preferably, the light source includes tunable, monochromatic light source such as, for example and not limitation, a tunable carbon dioxide laser or tunable infrared laser. Also preferably, the detector includes a photodetector and associated circuitry appropriate for receiving light pulses, and measuring the transmission time of the received light pulses from the light source and the differential diffraction of received light pulses having different wavelengths. Additionally, the detector preferably includes a processor for relating the measured differential diffraction to an environmental parameter being measured based, at least in part, on a known correlation between the measured differential diffraction and the environmental parameter.
In accordance with a method of using the optical-fiber based apparatus, the optical fiber-based sensor is positioned within an environment for which an environmental parameter is to determined. Because the optical fiber-based sensor is relatively flexible, is highly sensitive, and is not effected by electromagnetic radiation, the sensor may be positioned in a wide variety of environments (including, high-temperature environments) and in many orientations, and may even be imbedded in a wall manufactured of a solid material such as metal or concrete. Once positioned, the ends of the optical fiber-based sensor are operatively coupled to the light source and to the detector. Then, to make a measurement of a desired environmental parameter, the wavelength differential diffraction of light pulses is measured by the light source sequentially producing and directing light pulses into the optical fiber-based sensor and by the detector ascertaining, in concert with the light source, the transmission time of the light pulses from the light source to the detector. Next, the detector determines the value of the environmental parameter being measured by relating the measured differential diffraction to the environmental parameter based, at least in part, on a known correlation between the differential diffraction in the grating and the environmental parameter.
The optical fiber-based sensor is, preferably, formed according to a method of manufacture which includes exposing a silica-based, single mode, optical fiber to pulses of light from an infrared laser at periodic intervals, or locations, along the fiber""s length. Preferably, the light pulses of the infrared laser have a wavelength greater than approximately 2.0 xcexcm above which silica glass generally absorbs. Upon exposure to the infrared light, the refractive indices of the exposed areas of the optical fiber are heated and are altered, thereby producing diffraction gratings which differentially diffract light of differing wavelengths. It is believed that the thermal heating of the optical fiber by the infrared laser causes local densification of the optical fiber material and/or stress relief which creates changes to the refractive indices of the exposed areas. Unlike diffraction gratings produced by the exposure of an optical fiber to ultraviolet light, the diffraction gratings produced by the thermal-inducement of the present invention are stable and do not degrade when used in high-temperature environments for long periods of time.
Accordingly, it is an object of the present invention to provide optical fiber-based sensors which are stable at high temperatures.
Another object of the present invention is to provide optical fiber-based sensors which are suitable for use in extreme environments.
Still another object of the present invention is to provide optical fiber-based sensors which are not subject to degradation over time due to room temperature annealing of gratings.
Still another object of the present invention is to provide a method of manufacturing optical fiber-based sensors utilizing relatively low cost silica-based optical fibers.
Other objects, features, and advantages of the present invention will become apparent upon reading and understanding the present specification when taken in conjunction with the appended drawings.